Chemical Looping Combustion (CLC) refers to a technology for inherently separating carbon dioxide during a process without extra separation equipment, and can directly combust solid fuels instead of high-priced gaseous fuels and generate less thermal NOx and has high power generating efficiency. Therefore, CLC may be called a next-generation low-pollution and high-efficiency power generation technology.
In recent years, Solid Fuel Chemical Looping Combustion (SF-CLC), which can directly combust solid fuels instead of high-priced gaseous fuels and can inherently separate carbon dioxide during the solid fuel chemical looping combustion process without extra separation equipment, has been being researched.
A chemical looping combustor is mainly divided into an oxidation reactor and a reduction reactor. In the oxidation reactor, metal oxygen carriers react with oxygen in the air to produce a metallic oxide. The oxidation reactor transfers the metallic oxide to the reduction reactor.
In the reduction reactor, the metallic oxide transferred from the oxidation reactor reacts with solid fuels to produce carbon dioxide and steam, and the metal oxygen carriers reduced in the reduction reactor is transferred back to the oxidation reactor, such that the oxygen carriers are circulated between the reduction reactor and the oxidation reactor. Steam or carbon dioxide is supplied to the reduction reactor to fluidize the solid fuels, and the solid fuels are gasified by such fluidization gas.
FIG. 1 is a view to illustrate a concept of solid fuel chemical looping combustion technology.
Referring to FIG. 1, air flows into an oxidation reactor to fluidize metal solid particles, that is, oxygen carriers (M). In the oxidation reactor, the oxygen carriers M react with oxygen (O2) in the air to produce a metallic oxide (MO) as follows:M+0.5O2MO
The metallic oxide (MO) produced as in above Equation is transferred to a reduction reactor.
The metallic oxide (MO), solid fuels, and gases for fluidizing the solid fuels flow into the reduction reactor, and gasification reaction by steam and combustion reaction by the oxygen carriers (M) may occur simultaneously. The reaction may occur at high temperature of 600° C. to 1000° C. in the oxidation reactor and the reduction reactor.
First, as for the gasification reaction, the gases flowing into the reduction reactor may be steam or carbon dioxide (CO2). In the reduction reactor, the solid fuels are gasified by the fluidization gas. As the solid fuels, various fuels such as coal, coke, char, biomass, etc., may be used. In exemplary embodiments of the present invention, coal is used by way of an example.
In the reduction reactor, carbon included in the coal reacts with steam (or CO2) to produce CO and H2 (or CO) as in following Equation:C+H2O(or CO2)CO+H2(or CO):  <gasification reaction>
Thereafter, combustion reaction occurs in the reduction reactor. That is, CO and H2 (or CO) produced by the above-described gasification reaction reacts the metallic oxide transferred from the oxidation reactor to produce CO2, H2O, and oxygen carriers (M) as in following Equation:CO(or H2)+MOCO2(or H2O)+M:  <combustion reaction>
The oxygen carriers (M) produced by the combustion reaction is provided to the oxidation reactor, such that the oxygen carriers (M) are re-circulated.
In the related-art solid fuel chemical looping combustor which operates according to the principle described above with reference to FIG. 1, when solid fuels such as coal, biomass, waste, etc. are continuously supplied to the reduction reactor, gasification reaction occurs between the supplied solid fuels and the oxygen carriers and thus gases such as CO and H2 (or CO) are produced. Thereafter, combustible ingredients included in the gas, such as volatile matter, fixed carbon, etc., react with oxygen included in the oxygen carriers and are changed to gases such as CO2, CO, H2O, etc., and are discharged to the outside of the reactor. However, ash included in the solid fuels is accumulated in the reduction reactor. Accordingly, when the solid fuels are used, it is difficult to separate the oxygen carriers and ash from each other and thus the oxygen carriers and ash staying in the reduction reactor should be periodically or continuously removed.
Therefore, the related-art solid fuel chemical looping combustor uses low-priced oxygen carriers (e.g., iron ore, ilmenite, oxide scale or bauxite, or a compound thereof) which are disposable along with ash rather than using oxygen carriers containing high-priced metal components such as Ni, Co, Fe, Cu, Mn, Ce, etc. and a mixture thereof. However, when the low-priced oxygen carriers are discarded along with ash, a waste disposal cost is incurred, and also, since even the low-priced oxygen carriers should be continuously made up, there is a problem in the economic feasibility of oxygen carriers.
From among the oxygen carriers, the oxygen carriers containing high-priced metal components show excellent characteristics in comparison with low-priced oxygen carriers in terms of a reaction rate and an amount of oxygen that the oxygen carriers can transfer per unit weight. Therefore, even if an amount of solid circulation between the oxidation reactor and the reduction reactor is low, an operation is possible. However, when low-priced oxygen carriers are used for solid fuels, a reaction rate is low and thus a volume of the reactor should increase to guarantee sufficient residence time of the solids. In addition, the solid circulation rate between the oxidation reactor and the reduction reactor should increases to supply sufficient oxygen and thus the power cost increases.
Patent Document 1: US2011-0303875 A (Integrated oxidation, reduction, and gasification method for chemical looping syngas and energy production)